Hello,
I've only been using this programme for a few days and I've noticed that once I have my lineart and I try to use the "Contiguous Selection Tool" with the little wand icon it leaves a very small gap between the black lineart and the inner selected area.
Then when I fill it in and deselect I'm left with a white gap between the coloured area and the line art.

Is there a way to increase the threshold of it? And without using the feather tool since that makes it go out a few pixels in each direction with then often has it going out past the lineart. I know in SAI you could increase it so it bled a little into the black lines and selected them a bit too so not sure if something like that is possible here too.

Under the tool options the 'fuzziness' seems to be threshold feature.

I'd like to offer an alternate suggestion. The paintbucket/fill tool seems to work quite well for filling in lineart. Just use a blank layer under you line art, and as long as you don have 'limit to selected layer' checked, you can fill on your empty layer while still using your lineart as an outline.

There is also a lineart coloring feature in g'mic, but i dont like it very well. Sorry i dont remember what that one is called. David Devoy had a tutorial on it.